Satellite communications systems transmit content over large geographic regions. In a typical satellite communications system, end users interface with the system through user terminals. The user terminals communicate, via one or more satellites, with one or more gateway terminals. The gateway terminals may process and route data to and from one or more networks according to various protocols.
A typical hub-spoke spot beam satellite communications system may include a non-processing “bent pipe” spot beam satellite, many user terminals, and a smaller number of gateway terminals. Each user terminal may be connected to a network, such as the internet, via a series of communication links.
A forward link of a satellite communications system may consist of forward uplink transmissions from a gateway terminal to a satellite, a “bent pipe” repeater at the satellite, and forward downlink transmissions to a group of user terminals located in a common spot beam. The forward link may carry data from a gateway terminal to many different user terminals. The forward link may utilize, for example, time Division Multiplexing (TDM) and/or Frequency Division Multiplexing (FDM) of data into RF transmissions.
A return link of a satellite communications system may consist of return uplink transmissions from user terminals in a common spot beam to a satellite and return downlink transmissions from the satellite to a gateway terminal servicing the spot beam. Transmissions from many user terminals may utilize the return link simultaneously using various time and frequency division multiple access techniques. This may be referred to as Multi Frequency Time Division Multiple Access (MF-TDMA).
In typical MF-TDMA systems, each user terminal, or alternatively each burst transmitted by a user terminal, may be assigned one or more transmission slots consisting of a carrier center frequency and time interval. While user terminals are generally assigned to transmit on only one carrier at a time, the transition between carriers can be very dynamic (potentially burst to burst). Carrier and transmission frequency may be varied over a wide range of return link spectrum. This may be referred to as fast frequency hopping. With current frequency synthesizer technology, carrier separation of hundreds of MHz or more is readily achievable in low cost user terminals.
The large frequency separation between carriers can cause frequency gain variations within components of the satellite communications system. Within a user terminal, for example, the frequency gain of an indoor unit (IDU), an outdoor unit (ODU), and an inter facility link (IFL) can be significant. The IFL may be a connection, such as a coaxial cable, between the IDU and the ODU. The frequency gain within a low cost user terminal may vary by as much as 5-7 dB or more. Further, this variation may change with environmental conditions such as temperature. While the variation can be minimized by using precision electronic components and sophisticated data analysis techniques, these solutions increase costs and are not viable solutions for low cost user terminals commonly used in consumer applications.
The frequency gain variation may lead to error in return link power control. A typical return link power control system may measure a signal-to-noise ratio (SNR) of a received burst at a gateway and send an attenuation adjustment command to a user terminal. The system attempts to adjust effective isotropic radiated power (EIRP) at the user terminal to provide a desired SNR at the gateway. The EIRP at the user terminal may be adjusted using a programmable attenuator at the IDU or the ODU. The programmable attenuator may be a voltage variable attenuator (VVA) or a digital step attenuator.
Power control systems that adjust EIRP based on measured SNR, however, operate poorly in MF-TDMA systems. As an example, a system that uses an SNR error (difference between measured SNR and desired SNR) in a current burst to set EIRP may fail if the next burst is assigned on a different carrier frequency. The EIRP (or attenuation setting) may be accurate only for the previous carrier frequency. In such systems, frequency gain variations of 5-7 dB can lead to 5-7 dB of power control error. Another power control scheme may average the SNR error over a period of time and across all carrier frequencies to determine an average attenuation setting. Here, frequency gain variations of 5-7 dB may result in 2.5-3.5 dB of power control error at some frequencies. Further, the time it takes to average the SNR error may inhibit the power control system from responding to short term variations in uplink path loss such as rain fading.
Thus, there is a need for improved satellite link power control in MF-TDMA systems.